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S/MIME Working Group R. Housley
Internet Draft SPYRUS
expires in six months October 1997
Cryptographic Message Syntax
<draft-housley-smime-cms-00.txt>
Status of this Memo
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Abstract
This document describes the Cryptographic Message Syntax. This
syntax is used to digitally sign or encrypt arbitrary messages.
The Cryptographic Message Syntax is deriveded from PKCS #7 version
1.5. Wherever possible, backward compatibility is preserved;
however, changes were necessary to accomodate attribute certificate
transfer and key agreement techniques for key management.
Please send comments on this document to the ietf-smime@imc.com mail
list.
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1 Introduction
This document describes the Cryptographic Message Syntax. This
syntax is used to digitally sign or encrypt arbitrary messages.
The Cryptographic Message Syntax describes a general encapsulation
syntax for data protection. It supports digital signatures and
encryption. The syntax allows multiple enapsulation, so one
encapsulation envelope can be nested inside another. Likewise, one
party can digitally sign some previously encapsulated data. It also
allows arbitrary attributes, such as signing time, to be
authenticated along with the message content, and provides for other
attributes such as countersignatures to be associated with a
signature.
The Cryptographic Message Syntax can support a variety of
architectures for certificate-based key management, such as the one
defined by the PKIX working group.
The Cryptographic Message Syntax values are generated using ASN.1,
using BER-encoding. Values are typically represented as octet
strings. While many systems are capable of transmitting arbitrary
octet strings reliably, it is well known that many electronic-mail
systems are not. This document does not address mechanisms for
encoding octet strings for reliable transmission in such
environments.
2 General Overview
The Cryptographic Message Syntax is general enough to support many
different content types. This document defines three: data, signed
data, and enveloped data. Other content types may be added in the
future, and additional content types can be defined outside this
document.
The Cryptographic Message Syntax exports one content type,
ContentInfo, as well as the various object identifiers.
As a general design philosophy, content types permit single pass
processing using indefinite-length Basic Encoding Rules (BER)
encoding. Single-pass operation is especially helpful if content is
large, stored on tapes, or is "piped" from another process. Single-
pass operation has one significant drawback; it is difficult to
perform encode operations using the Distinguished Encoding Rules
(DER) encoding in a single pass since the lengths of the various
components may not be known in advance. Since DER encoding is
required by the signed-data content type, an extra pass may be
necessary when a content type other than data is encapsulated.
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3 General Syntax
The Cryptographic Message Syntax associates a content type with a
content. The syntax shall have ASN.1 type ContentInfo:
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
The fields of ContentInfo have the following meanings:
contentType indicates the type of content. It is an object
identifier, which means it is a unique string of integers assigned
by the authority that defines the content type.
content is the content. The field is optional, althought it is
generally present. In the rare cases where it is absent, the
intended value must be supplied by other means.
in 3 The methods below assume that the type of content can be
determined uniquely by contentType, so the type defined along with
the object identifier should not be a CHOICE type.
When a ContentInfo value is encapsulated within signed-data, a
message-digest algorithm is applied to the contents octets of the
DER encoding of the content field. When a ContentInfo value is
encapsulated within enveloped-data, a content-encryption algorithm
is applied to the contents octets of a definite-length BER
encoding of the content field.
The optional omission of the content field makes it possible to
construct "external signatures." In the case of external
signatures, the content being signed would be absent from the
encapsulated ContentInfo value included in the signed-data content
type.
4 Data Content Type
The data content type is identified by the following object
identifier:
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
US(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
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The data content type is just an octet string. It shall have ASN.1
type Data:
Data ::= OCTET STRING
The data content type is intended to refer to arbitrary octet
strings, such as ASCII text files; the interpretation is left to the
application. Such strings need not have any internal structure
(although they may; they could even be DER encodings).
5 Signed-data Content Type
The signed-data content type consists of a content of any type and
zero or more signature values. Any type of content can be signed by
any number of signers in parallel.
The typical application of the signed-data content type represents
one signer's digital signature on content of the data content type.
Another typical application disseminates certificates and certificate
revocation lists (CRLs).
The process by which signed data is constructed involves the
following steps:
1. For each signer, a message digest, or hash value, is computed
on the content with a signer-specific message-digest algorithm. If
two signers employ the same message digest algorithm, then the
message digest need be computed for only one of them. If the
signer is authenticating any information other than the content
(see Section 5.2), the message digest of the content and the other
information are digested with the signer's message digest
algorithm, and the result becomes the "message digest."
2. For each signer, the message digest is digitally signed using
the signer's private key.
3. For each signer, the signature value and other signer-specific
information are collected into a SignerInfo value, as defined in
Section 5.2. Certificates and CRLs for each signer, and those not
corresponding to any signer, are collected in this step.
4. The message digest algorithms for all the signers and the
SignerInfo values for all the signers are collected together with
the content into a SignedData value, as defined in Section 5.1.
A recipient independently computes the message digest. This message
digest and the signer's public key are used it to validate the
signature value. The signer's public key is either contained in a
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certificate included in the signer information, or is referenced by
an issuer distinguished name and an issuer-specific serial number
that uniquely identify the certificate containing the public key.
This section is divided into four parts. The first part describes the
top-level type SignedData, the second part describes the per-signer
information type SignerInfo, and the third and fourth parts describe
the message digest calculation and signiture generation processes.
5.1 SignedData Type
The signed-data content type is identified by the following object
identifier:
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
US(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
The signed-data content type shall have ASN.1 type SignedData:
SignedData ::= SEQUENCE {
version Version,
digestAlgorithms DigestAlgorithmIdentifiers,
contentInfo ContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT CRLs OPTIONAL,
signerInfos SignerInfos }
DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier
SignerInfos ::= SET OF SignerInfo
The fields of type SignedData have the following meanings:
version is the syntax version number. If no attribute certificates
are present in the certificates field, then the value of version
shall 1; however, if attribute certificates are present, then the
value of version shall be 3.
digestAlgorithms is a collection of message digest algorithm
identifiers. There may be any number of elements in the
collection, including zero. Each element identifies the message
digest algorithm, along with any associatied parameters, used by
one or more signer. The collection is intended to list the message
digest algorithms employed by all of the signers, in any order, to
facilitate one-pass signature verification. The message digesting
process is described in Section 5.3.
contentInfo is the content that is signed. It can have any type.
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certificates is a collection of certificates. It is intended that
the set of certificates be sufficient to contain chains from a
recognized "root" or "top-level certification authority" to all of
the signers in the signerInfos field. There may be more
certificates than necessary, and there may be certificates
sufficient to contain chains from two or more independent top-
level certification authorities. There may also be fewer
certificates than necessary, if it is expected recipients have an
alternate means of obtaining necessary certificates (e.g., from a
previous set of certificates). If no attribute certificates are
present in the collection, then the value of version shall 1;
however, if attribute certificates are present, then the value of
version shall be 3.
crls is a collection of certificate revocation lists (CRLs). It is
intended that the set contain information sufficient to determine
whether or not the certificates in the certificates field are
valid, but such correspondence is not necessary. There may be more
CRLs than necessary, and there may also be fewer CRLs than
necessary.
signerInfos is a collection of per-signer information. There may
be any number of elements in the collection, including zero.
In the degenerate case where there are no signers, the ContentInfo
value being "signed" is irrelevant. In this case, the content type
within the ContentInfo value being "signed" should be data, and the
content field of the ContentInfo value should be omitted.
5.2 SignerInfo Type
Per-signer information is represented in the type SignerInfo:
SignerInfo ::= SEQUENCE {
version Version,
issuerAndSerialNumber IssuerAndSerialNumber,
digestAlgorithm DigestAlgorithmIdentifier,
authenticatedAttributes [0] IMPLICIT Attributes OPTIONAL,
signatureAlgorithm signatureAlgorithmIdentifier,
signature SignatureValue,
unauthenticatedAttributes [1] IMPLICIT Attributes OPTIONAL }
SignatureValue ::= OCTET STRING
The fields of type SignerInfo have the following meanings:
version is the syntax version number. It shall always be 1.
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issuerAndSerialNumber specifies the signer's certificate (and
thereby the signer's public key) by issuer distinguished name and
issuer-specific serial number.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the signer. The message digest is
computed over the the content and authenticated attributes, if
present. The message digest algorithm should be among those listed
in the digestAlgorithms field of the superior SignerInfo value.
The message digesting process is described in Section 5.3.
authenticatedAttributes is a collection of attributes that are
signed. The field is optional, but it must be present if the
content type of the ContentInfo value being signed is not data. If
the field is present, it must contain, at a minimum, two
attributes:
A PKCS #9 content-type attribute having as its value the
content type of the ContentInfo value being signed.
A PKCS #9 message-digest attribute, having as its value the
message digest of the content.
Other attribute types that might be useful here, such as
signing time, are also defined in PKCS #9.
signatureAlgorithm identifies the signature algorithm, and any
associated parameters, used by the signer to genrate the digital
signature.
signature is the result of digital signature generation, using the
message digest and the signer's private key.
unauthenticatedAttributes is a collection of attributes that are
not signed. The field is optional. Attribute types that might be
useful here, such as countersignatures, are defined in PKCS #9.
5.3 Message Digest Calculation Process
The message digest calculation process computes a message digest on
either the content being signed or the content together with the
signer's authenticated attributes. In either case, the initial input
to the message digest calculation process is the "value" of the
content being signed. Specifically, the initial input is the content
octets of the DER encoding of the content field of the ContentInfo
value to which the signing process is applied. Only the contents
octets of the DER encoding of that field are input to the message
digest algoritm, not the identifier octets or the length octets.
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The result of the message digest calculation process depends on
whether the authenticatedAttributes field is present. When the field
is absent, the result is just the message digest of the content as
described above. When the field is present, however, the result is
the message digest of the complete DER encoding of the Attributes
value containted in the authenticatedAttributes field. Since the
Attributes value, when the field is present, must contain as
attributes the content type and the message digest of the content,
those values are indirectly included in the result. A separate
encoding of the authenticatedAttributes field is performed for
message digest calculation. The IMPLICIT [0] tag in the
authenticatedAttributes field is not used for the DER encoding,
rather an EXPLICIT SET OF tag is used. That is, the DER encoding of
the SET OF tag, rather than of the IMPLICIT [0] tag, is to be
included in the message digest calculation along with the length and
contents octets of the Attributes value.
When the content being signed has content type data and the
authenticatedAttributes field is absent, then just the value of the
data (e.g., the contents of a file) is input to the message digest
calculation. This has the advantage that the length of the content
being signed need not be known in advance of the encryption process.
Although the identifier octets and the length octets are not included
in the message digest calculation, they are still protected by other
means. The length octets are protected by the nature of the message
digest algorithm since it is computationally infeasible to find any
two distinct messages of any length that have the same message
digest.
The fact that the message digest is computed on part of a DER
encoding does not mean that DER is the required method of
representing that part for data transfer. Indeed, it is expected that
some implementations will store objects in forms other than their DER
encodings, but such practices do not affect message digest
computation.
5.4 Message Signature Generation Process
The input to the signature generation process includes the result of
the message digest calculation process and the signer's private key.
The details of the signature generation depend on the signature
algorithm deployed. The object identifier, along with any
parameters, that specifies the signature algorithm deployed by the
signer is carried in the signatureAlgorithm field. The signature
value generated by the signer is encoded as an OCTET STRING and
carried in the signature field.
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6 Enveloped-data Content Type
The enveloped-data content type consists of an encrypted content of
any type and encrypted content-encryption keys for one or more
recipients. The combination of the encrypted content and one
encrypted content-encryption key for a recipient is a "digital
envelope" for that recipient. Any type of content can be enveloped
for any number of recipients.
The typical application of the enveloped-data content type will
represent one or more recipients' digital envelopes on content of the
data or signed-data content types.
Enveloped-data is constructed by the following steps:
1. A content-encryption key for a particular content-encryption
algorithm is generated at random.
2. The content-encryption key is encrypted for each recipient.
The details of this encryption depend on the key management
algorithm used, but three genral techniques are supported:
key transport: the content-encryption key is encrypted in the
recipient's public key;
key agreement: the recipient's public key and the sender's
private key are used to generate a pairwise symmetric key, then
the content-encryption key is encrypted in the pairwise
symmetric key; and
mail list keys: the content-encryption key is encrypted in a
previously distributed symmetric key.
3. For each recipient, the encrypted content-encryption key and
other recipient-specific information are collected into a
RecipientInfo value, defined in Section 6.2.
4. The content is encrypted with the content-encryption key.
Content encryption may require that the content be padded to a
multiple of some block size; see Section 6.3.
5. The RecipientInfo values for all the recipients are collected
together with the encrypted content into a EnvelopedData value as
defined in Section 6.1.
A recipient opens the envelope by decrypting the one of the encrypted
content-encryption keys and decrypting the encrypted content with the
recovered content-encryption key.
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This section is divided into four parts. The first part describes the
top-level type EnvelopedData, the second part describes the per-
recipient information type RecipientInfo, and the third and fourth
parts describe the content-encryption and key-encryption processes.
6.1 EnvelopedData Type
The enveloped-data content type is identified by the following object
identifier:
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
US(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
The enveloped-data content type shall have ASN.1 type EnvelopedData:
EnvelopedData ::= SEQUENCE {
version Version,
originatorInfo [0] OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
encryptedContentInfo EncryptedContentInfo }
OriginatorInfo ::= SEQUENCE {
certs [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT CertificateRevocationLists OPTIONAL,
ukms [2] UserKeyingMaterials OPTIONAL }
RecipientInfos ::= SET OF RecipientInfo
EncryptedContentInfo ::= SEQUENCE {
contentType ContentType,
contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }
EncryptedContent ::= OCTET STRING
The fields of type EnvelopedData have the following meanings:
version is the syntax version number. If originatorInfo is
present, then version shall be 2. If any of the RecipientInfo
structures included have a version of 2, then the version shall be
2. If originatorInfo is absent and all of the RecipientInfo
structures are version 0, then version shall be 0.
recipientInfos is a collection of per-recipient information. There
must be at least one element in the collection.
encryptedContentInfo is the encrypted content information.
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The fields of type EncryptedContentInfo have the following meanings:
contentType indicates the type of content.
contentEncryptionAlgorithm identifies the content-encryption
algorithm, and any associated parameters, used to encrypt the
content. The content-encryption process is described in Section
6.3. The same algorithm is used for all recipients.
encryptedContent is the result of encrypting the content. The
field is optional, and if the field is not present, its intended
value must be supplied by other means.
The recipientInfos field comes before the encryptedContentInfo field
so that an EnvelopedData value may be preoceesed in a single pass.
6.2 RecipientInfo Type
Per-recipient information is represented in the type RecipientInfo:
RecipientInfo ::= SEQUENCE {
version Version,
rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] RecipientKeyIdentifier,
mlKeyId [1] MailListKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
MailListKeyIdentifier ::= SEQUENCE {
kekIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
EncryptedKey ::= OCTET STRING
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The fields of type RecipientInfo have the following meanings:
version is the syntax version number. If the RecipientIdentifier
is the CHOICE issuerAndSerialNumber, then the version shall be 0.
If the RecipientIdentifier is either the CHOICE rKeyId or mlKeyId,
then the version shall be 2.
rid specifies the recipient's certificate or key that was used by
the sender to protect the content-encryption key.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key for the recipient. The key-encryption process is
described in Section 6.4.
encryptedKey is the result of encrypting the content-encryption
key for the recipient.
The RecipientIdentifier is a CHOICE with three alternatives. The
first two alternatives, issuerAndSerialNumber and rKeyId, specifies
the recipient's certificate, and thereby the recipient's public key.
The rKeyId alternative may optionally specify other parameters
needed, such as the date. If the recipient's certificate contains a
key transport public key, then the content-encryption key is
encrypted with the recipient's public key. If the recipient's
certificate contains a key agreement public key, then a pairwise
symmetric key is established and used to encrypt the content-
encryption key. The third alternative, mlKeyId, specifies a
symmetric key encryption key that was previously distributed to the
sender and recipient.
The fields of type RecipientKeyIdentifier have the following
meanings:
subjectKeyIdentifier identifies the recipient's certificate by the
X.509 subjectKeyIdentifier extension value.
date is optional. When present, the date specifies which of the
recipient's UKMs was used by the sender.
other is optional. When present, this field contains additional
information used by the recipient to locate the keying material
used by the sender.
The fields of type MailListKeyIdentifier have the following meanings:
kekIdentifier identifies the key-encryption key that was
previously distributed to the sender and the recipient.
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date is optional. When present, the date specifies a single key-
encryption key from a set that was previously distributed to the
sender and the recipient.
other is optional. When present, this field contains additional
information used by the recipient to locate the keying material
used by the sender.
6.3 Content-encryption Process
The input to the content-encryption process is the "value" of the
content being enveloped. Specifically, the input is the content
octets of a definite-length BER encoding of the content field of the
ContentInfo value. Only the content octets of the BER encoding are
encrypted, not the identifier octets or length octets; those other
octets are not included.
When the content being enveloped has content type data, then just the
value of the data (e.g., the contents of a file) is encrypted. This
has the advantage that the length of the content being encrypted need
not be known in advance of the encryption process.
The identifier octets and the length octets are not encrypted. The
length octets may be protected implicitly by the encryption process,
depending on the encryption algorithm. The identifier octets are not
protected at all, although they can be recovered from the content
type, assuming that the content type uniquely determines the
identifier octets. Explicit protection of the identifier and length
octets requires that the signed-data content type be employed prior
to enveloping.
A definite-length BER encoding is used to ensure that the bit
indicating whether the length is definite or indefinite is not
recorded in the enveloped-data content type. Definite-length encoding
is more appropriate for simple types such as octet strings, so
definite-length encoding is chosen.
Some content-encryption algorithms assume the input length is a
multiple of k octets, where k is greater than one. For such
algorithms, the input shall be padded at the trailing end with
k-(l mod k) octets all having value k-(l mod k), where l is the
length of the input. In other words, the input is padded at the
trailing end with one of the following strings:
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01 -- if l mod k = k-1
02 02 -- if l mod k = k-2
.
.
.
k k ... k k -- if l mod k = 0
The padding can be removed unambiguously since all input is padded,
including input values that are already a multiple of the block size,
and no padding string is a suffix of another. This padding method is
well-defined if and only if k is less than 256.
6.4 Key-encryption Process
The input to the key-encryption process -- the value supplied to the
recipient's key-encryption algorithm --is just the "value" of the
content-encryption key.
7 Useful Types
This section defines types that are used other places in the
document. The types are not listed in any particular order.
7.1 CertificateRevocationLists
The CertificateRevocationLists type gives a set of certificate
revocation lists (CRLs). It is intended that the set contain
information sufficient to determine whether the certificates with
which the set is associated are revoked or not. However, there may
be more CRLs than necessary, or there may be fewer than necessary.
The definition of CertificateRevocationList is imported from X.509.
CertificateRevocationLists ::= SET OF CertificateRevocationList
7.2 ContentEncryptionAlgorithmIdentifier
The ContentEncryptionAlgorithmIdentifier type identifies a content-
encryption algorithm such as DES. A content-encryption algorithm
supports encryption and decryption operations. The encryption
operation maps an octet string (the message) to another octet string
(the ciphertext) under control of a content-encryption key. The
decryption operation is the inverse of the encryption operation.
Context determines which operation is intended.
The definition of AlgorithmIdentifier is imported from X.509.
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
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7.3 DigestAlgorithmIdentifier
The DigestAlgorithmIdentifier type identifies a message-digest
algorithm. Examples include SHA-1, MD2, and MD5. A message-digest
algorithm maps an octet string (the message) to another octet string
(the message digest).
The definition of AlgorithmIdentifier is imported from X.509.
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
7.4 SignatureAlgorithmIdentifier
The SignatureAlgorithmIdentifier type identifies a signture
algorithm. Examples include DSS and RSA. A signature algorithm
supports signature generation and verification operations. The
signature generation operation uses the message digest and the
signer's private key to generate a signutre value. The signature
verification operation uses the message digest and the signer's
public key to determine whether or not a signutre value is valid.
Context determines which operation is intended.
The definition of AlgorithmIdentifier is imported from X.509.
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
7.5 CertificateChoices
The CertificateChoices type gives either a PKCS #6 extended
certificate, an X.509 certificate, or an X.509 attrinute certificate.
The PKCS #6 extended certificate is obsolete. It is included for
backwards compatibility, and its use should be avoided.
The definitions of Certificate and AttributeCertificate are imported
from X.509.
CertificateChoices ::= CHOICE {
certificate Certificate, -- See X.509
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
attrCert [1] AttributeCertificate } -- See X.509 and X9.57
7.6 CertificateSet
The CertificateSet type provides a set of certificates. It is
intended that the set be sufficient to contain chains from a
recognized "root" or "top-level certification authority" to all of
the sender certificates with which the set is associated. However,
there may be more certificates than necessary, or there may be fewer
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than necessary.
The precise meaning of a "chain" is outside the scope of this
document. Some applications may impose upper limits on the length of
a chain; others may enforce certain relationships between the
subjects and issuers of certificates within a chain.
CertificateSet ::= SET OF CertificateChoices
7.7 IssuerAndSerialNumber
The IssuerAndSerialNumber type identifies a certificate, and thereby
an entity and a public key, by the distinguished name of the
certificate issuer and an issuer-specific certificate serial number.
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
7.8 KeyEncryptionAlgorithmIdentifier
The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption
algorithm used to encrypt a content-encryption key. The encryption
operation maps an octet string (the key) to another octet string (the
encrypted key) under control of a key-encryption key. The decryption
operation is the inverse of the encryption operation. Context
determines which operation is intended.
The details of encryption and decryption depend on the key management
algorithm used. Key transport, key agreement, and previously
distributed symmetric key-encrypting keys are supported.
The definition of AlgorithmIdentifier is imported from X.509.
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
7.9 Version
The Version type gives a syntax version number, for compatibility
with future revisions of this document.
Version ::= INTEGER
7.10 UserKeyingMaterial
The UserKeyingMaterial type gives a syntax user keying material
(UKM). Some key management algorithms require UKMs. The sender
provides a UKM for the specific key management algorithm.
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The definition of AlgorithmIdentifier is imported from X.509.
UserKeyingMaterial ::= SEQUENCE {
algorithm AlgorithmIdentifier,
ukm OCTET STRING }
7.11 UserKeyingMaterials
The UserKeyingMaterial type provides a set of user keying materials
(UKMs). This allows the sender to provide a UKM for each key
management algorithm that requires one.
UserKeyingMaterials ::= SET OF UserKeyingMaterial
7.12 OtherKeyAttribute
The OtherKeyAttribute type gives a syntax for the inclusion of other
key attributes that permit the recipient to select the key used by
the sender. The attribute object identifier must be registered along
with the syntax of the attribute itself. Use of this structure
should be avoided since it may impede interoperability.
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
References
To be supplied.
Security Considerations
The Cryptographic Message Syntax provides a method for digitally
signing data and encrypting data.
Implementations must protect the signer's private key. Compromise of
the signer's private key permits masquerade.
Implementations must protect the key management private key and the
content-encryption key. Compromise of the key management private key
may result in the disclosure of all messages protected with that key.
Similarly, compromise of the content-encryption key may result in
disclosure of the encrypted content.
Housley [Page 17]
INTERNET DRAFT October 1997
Author Address
Russell Housley
SPYRUS
PO Box 1198
Herndon, VA 20172
USA
housley@spyrus.com
Housley [Page 18]
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